1. Field of the Invention
The invention relates to improved insulative coatings for electrical steels, and more particularly to insulative coatings characterized by a hard, smooth, glassy nature, improved moisture resistance, excellent space factor characteristics and which improve the magnetic characteristics of the electrical steels to which they are applied.
2. Description of the Prior Art
As used herein and in the claims the terms "electrical steel" and "silicon steel" relate to an alloy, the typical composition of which by weight percent falls within the following:
______________________________________ Carbon 0.060% maximum Silicon 4% maximum Sulfur or Selenium 0.03% maximum Manangese 0.02%-0.4% Aluminum 0.4% maximum Iron Balance ______________________________________
While the insulative coatings of the present invention are applicable to carbon steels for electrical uses, non-oriented silicon steels and silicon steels having various orientations, they will, for purposes of an exemplary showing, be described with respect to their application to cube-on-edge oriented silicon steel. Such silicon steel is well known in the art and is characterized by the fact that the body-centered cubes making up the grains or crystals are oriented in a position designated (110)[001] in accordance with Miller's indices. Cube-on-edge oriented sheet gauge silicon steel has many uses, an exemplary one of which is the manufacture of laminated magnetic cores for power transformers and the like. In such an application, the magnetic characteristics of the cube-on-edge oriented silicon steel are important, and primary among these are core loss, interlamination resistivity, space factor and magnetostriction.
Prior art workers have recognized that the magnetic characteristics of cube-on-edge oriented silicon steel, and particularly those mentioned above, are enhanced if the silicon steel is provided with a surface film or glass. In the commercial manufacture of cube-on-edge oriented silicon steel an annealing separator is used during the final anneal to which the silicon steel is subjected (i.e. that anneal during which the cube-on-edge orientation is achieved). When an appropriate annealing separator is used, as for example magnesia or magnesia-containing annealing separators, a glass film is formed upon the surfaces of the silicon steel. This glass or film is generally referred to in the industry as a "mill glass". Heretofore, much work has been done toward the improvement of mill glass, as is exemplified in U.S. Pat. No. 2,385,332 and 3,615,918.
In some applications it is desirable to have an applied insulative coating rather than, or in addition to, the mill glass formed during the high temperature, orientation-determining anneal. This has led to the development of phosphate coatings such as those taught in U.S. Pat. No. 2,501,846; 2,492,095 and the copending application in the name of the present inventor, Ser. No. 237,344, filed Mar. 23, 1972 and entitled INSULATIVE COATINGS FOR ELECTRIC STEELS.
Prior art workers have also devoted much attention to the improvement of applied insulative coatings. A number of magnesium phosphate based coatings and aluminum phosphate based coatings have been developed, as exemplified by U.S. Pat. No. 2,743,203; 3,151,000; 3,594,240 and 3,687,742.
U.S. Pat. No. 3,649,372 teaches a reagent for forming an applied insulative coating, the major component of which is mono-basic magnesium phosphate. The reagent also includes aluminum nitrate and/or aluminum hydroxide together with chromic anhydride.
Belgian Patent No. 789,262 teaches an applied insulative coating involving the use of mono-aluminum phosphate solution, colloidal silica solution and chromic acid or magnesium chromate. The coating of this reference is intended to exert tension on the silicon steel strip to improve various ones of its magnetic properties. U.S. Pat. No. 3,594,240 and 3,687,742, mentioned above, also teach the benefits of a tension-imparting film.
The present invention is directed to improved applied coatings which may be used in addition to or in lieu of a mill glass. The invention is based upon the discovery that excellent insulative and tension-imparting applied coatings can be produced from an aqueous solution containing appropriate relative concentrations of A1.sup.+.sup.+.sup.+, Mg.sup.+.sup.+ and H.sub.2 PO.sub.4.sup.- as will be taught hereinafter. If the curing of the coatings is accomplished in a conventional roller hearth furnace for thermal flattening of the strip, colloidal silica may be added to the coating solutions to prevent adherence of the coatings to the furnace rolls. Chromic anhydride may also be added to the coating solutions in a specified amount to improve their wettability, to enhance the moisture resistance of the final coatings and to improve the interlaminar resistivity after stress relief annealing. Upon curing, a hard, glassy, smooth-surfaced, tension imparting film or glass is formed having excellent space factor characteristics and improving the magnetic characteristics of the silicon steel. The coatings of the present invention can be cured at a temperature lower than those required by the usual phosphate coatings.